The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. As illustrated in FIG. 1, an E-UTRAN typically comprises user equipments (UE) 150 wirelessly connected to radio base stations (RBS) 100, commonly referred to as eNodeB. The eNodeB serves one or more areas referred to as cells 110.
Mobile user positioning is the process of determining UE coordinates in space. Once the coordinates are available, the position can be mapped to a certain place or location. The mapping function and the delivery of the location information on request are parts of the location service which is required for the basic emergency services. Services that further exploit the location knowledge or that are based on location knowledge to offer customers some additional value, are referred to as location-aware and location-based services, respectively.
There exist a variety of positioning techniques in wireless communications networks, differing in their accuracy, implementation cost, complexity, applicability in different environments, etc. In existing networks, the most common are UE assisted solutions where a serving mobile location center 120 (SMLC in GSM and UMTS, enhanced SMLC (eSMLC) in LTE) calculates the UE position based on measurements reported by the UE. The SMLC/eSMLC 120 is either a separate network element (as illustrated in FIG. 1) or an integrated functionality in the RBS. Among such methods, Assisted Global Positioning System (A-GPS) typically provides the best accuracy. Combining the mobile technology and GPS, A-GPS enhances the UE receiver sensitivity by providing orbit and other data to the UE. Drawbacks of A-GPS is that a GPS-equipped UE is required, and that it doesn't function in certain environments such as tunnels, indoor areas and dense urban areas. Therefore other complementing methods for positioning are needed. These methods use measurements of the time difference of arrival (TDOA) of signals between the cellular antenna and the UE. In UMTS observed TDOA (OTDOA) is used. In GSM a variant called Enhanced Observed Time Difference (E-OTD) is used.
The technique currently adopted for LTE-based positioning is OTDOA. OTDOA is a multi-lateration based technique estimating TDOA of signals received from three or more sites. To enable positioning, the UE should be able to detect signals from at least three geographically dispersed RBS. This implies that the signals need to have high enough signal-to-interference ratios (SINR). Furthermore, the signals need to be transmitted frequently enough to meet the service delay requirements. In order to meet the accuracy requirements, the signals may need to be accumulated over multiple sub frames.
There is currently no completely standardized positioning method for LTE and therefore there is no existing reference solution. To enable positioning measurements in LTE, a straightforward solution would be to measure standardized signals that are always transmitted from LTE RBS, e.g. synchronization signals (SS) or cell-specific reference signals (RS). SS and cell-specific RS (CRS) are physical signals used to support physical-layer functionality and they do not carry any information from the Medium Access Control (MAC) layer. Both signals are transmitted according to a pre-defined pattern, i.e. in selected subcarriers and time slots, and the pattern is typically relatively sparse.
In LTE, SS are transmitted in downlink and are primarily used in the cell search procedure, i.e. for the UE to identify a cell and synchronize to it in downlink in order to read the broadcast channel information. As shown in FIG. 2a, SS are transmitted in sub frame 0, 220, and sub frame 5, 230, of a radio frame 210. A SS consists of Primary SS (PSS) 240 and Secondary SS (SSS) 250. First, a cell identity is read from PSS, and then the cell identity group is read from SSS. The cell identity can then be used to determine the CRS sequence and its allocation in the time-frequency grid. In FIG. 2b, it is shown that the SS occupy 62 resource elements in the center of the allocated bandwidth.
CRS are transmitted over the entire system bandwidth and in every sub frame, i.e. more frequently than SS. In normal sub frames with a normal cyclic prefix where each time slot comprises seven OFDM symbols, CRS are transmitted on the resource elements (RE) shown in FIG. 3a, illustrating the RE 310 of a time-frequency resource grid for one sub frame 311 in time and 12 subcarriers 312 in frequency (the number of subcarriers corresponding to a physical resource block (PRB)). FIG. 3a shows the RE used for CRS 313 in a system with a single transmit antenna. In such a system, up to six different shifts in frequency (frequency reuse factor =6) and 504 different signals can be used for the CRS. With two transmit antennas, the maximum frequency reuse factor reduces to three, which is shown in FIGS. 3b and 3c. FIG. 3b illustrates the time frequency resource grid for a first antenna port, indicating the RE used for CRS for this first antenna port 313 (similar to FIG. 3a) as well as the RE reserved for CRS for the second antenna port 314. FIG. 3c illustrates the time frequency resource grid for the second antenna port, indicating the RE used for CRS for this second antenna port 316, corresponding to the reserved RE 314 in FIG. 3b, as well as the RE reserved for CRS for the first antenna port 315. With four transmit antennas, the possibilities are even more limited as shown in FIG. 3d, illustrating the time frequency resource grid for a first antenna port out of four. In FIG. 3d, the RE used for CRS for this first antenna port 317 as well as the RE reserved for CRS for the other three antenna ports 318 are indicated. Other CRS patterns are defined for sub frames with extended cyclic prefix and for multicast broadcast single frequency network (MBSFN) sub frames.
However, it has been shown that using SS and CRS for positioning without interference management would result in positioning coverage problems due to low SINR and/or insufficient number of strong signals from different RBS. The problem is particularly relevant for synchronized networks or networks with high data load, as there is a high probability of parallel transmissions in multiple cells on the RE used for CRS or SS which leads to high interference. Furthermore, the SS transmission frequency is not sufficient for the positioning requirements.
To improve positioning measurements and address the hearability problem, it has been proposed in 3GPP to introduce positioning RS (PRS), which could be designed according to transmission patterns characterized by a lower collision probability. The transmission periodicity for PRS is under discussion. In general, PRS may or may not be transmitted in multiple consecutive sub frames and the periodicity can be configured statically or semi-statically.
In respect to the frequency dimension, given a PRS transmission pattern per PRB, the simplest solution would be to repeat the same pattern in all PRB of the same sub frame, i.e., over the entire bandwidth. Transmitting PRS over a large bandwidth generally improves positioning accuracy due to a higher measurement resolution and a lower probability of being in unfavourable frequency-selective fading conditions. The drawback is that a large bandwidth gives a high UE complexity. Furthermore, a smaller bandwidth may be sufficient to achieve the required accuracy, and using the entire bandwidth is then a waste of resources.
At a high system load, there is no gain in introducing the new PRS without interference coordination. One of the approaches for reducing interference is to transmit PRS during low-interference sub frames (LIS) in which PDSCH transmissions are suppressed. FIG. 4 illustrates an example of a LIS 400 for a single cell with a possible PRS pattern, where the data transmission is suppressed in all PRB 440 of the entire transmission bandwidth of the cell 450. In the LIS 400, there are RE used for PRS 410, RE used for control signalling 420, but the rest of the REs are free from data transmission 430. To achieve an even higher interference reduction, LIS can be aligned among the cells. For the LIS alignment, inter-cell coordination may or may not be needed, depending e.g. on if LIS occurrences are configured statically or dynamically. FIG. 5 illustrates an example with aligned LIS 500 for a synchronized network with three-cell sites and a frequency reuse of three for the PRS. In the frequency dimension, only one PRB per cell is illustrated. The RE used for PRS in the current cell 520 are indicated in each cell's time-frequency resource grid, together with the RE used for PRS in some other cell 530. Cells within the same re-use group (e.g. cell(1,1) and cell(2,1)) will have colliding PRS 510.
In LTE networks, some sub frames can be configured to be MBSFN sub frames. Such sub frames are utilized for multicast/broadcast transmissions such as Mobile TV, and will not be utilized initially when the service is not supported. These sub frames could thus also be considered as low-interference sub frames during which transmission of PRS would be allowed. This is only a feasible solution in the release which does not support Multimedia Broadcast Multicast Service (MBMS), and is thus not a future proof solution.
According to another approach, currently used for UMTS and discussed for LTE, special periods called idle periods downlink (IPDL) in a cell (cell IPDL) or site (site IPDL) may be used for PRS transmission. No transmission occurs during the IPDL. This approach has been used in UMTS networks, and due to the radio technology specifics it has only been considered for the entire system bandwidth. Using IPDL for the entire system bandwidth may result in inefficient resource utilization for technologies that admit larger system bandwidths and allow for transmissions over smaller parts of the bandwidth.